Susceptors are commonly used to enhance microwave cooking techniques and apply those techniques to a wider variety of food products. They are usually incorporated in disposable food containers.
A typical susceptor includes a thin layer of microwave-interactive material, such as aluminum, deposited on a substrate, usually a plastic film. Most often, the susceptor is bonded to a sheet of paper that forms part of a bag or box.
A common problem associated with susceptors currently used in disposable packaging is their cracking and breakup during the cooking process. This problem, and the attendant risk of contamination of the food within the disposable packaging, are typically solved by overlaying the susceptor with a sheet of microwave-permeable and resilient material, or placing it between two or more layers of the material forming the food packaging.
This loss of the structural integrity of the susceptor is believed to be caused largely by differing coefficients of thermal expansion of the aluminum layer, the polyester substrate, the paper backing, and the adhesives that bond these layers together. The problem is exacerbated by the propensity of many plastic materials to expand significantly during the early stages of cooking, and then to shrink as the temperature increases beyond a certain level.
The breakup of a susceptor can be reduced by maintaining strict manufacturing tolerances during its production, and by judicious selection and uniform application of an adhesive. However, there are practical limitations on the degree to which manufacturing tolerances can be maintained during high volume production. Even minor variations in material thickness, for example, can trigger cracking and breakup of the susceptor.
In most instances the cracking and breakup of the susceptor is thought to start in the thin metallic layer of microwave-interactive material. These cracks begin to form early in the heating process, when the substrate expands at a considerably faster rate than the metallic layer deposited on it. However, as the temperature of the susceptor rises beyond a certain level, the substrate begins to shrink, while the metallic layer continues to expand. The resulting thermal stresses in the interface between the metallic layer and the substrate, as well as within the substrate, tend to propagate the random cracks in the metallic layer. It is thought that these cracks, as they become larger, cause corresponding cracks in the adjacent substrate. The cracks in the substrate may then be further enlarged due to internal stresses within the substrate.
It is believed that the breakup of the susceptor greatly reduces the heating effect of the microwave-interactive layer. It is theorized that this phenomenon is due to the tendency of the cracks to disrupt eddy currents in the susceptor that cause heating through I.sup.2 R losses. The breakup of the susceptor therefore has a thermostatic effect, decreasing the generation of heat at the temperature at which breakup occurs. This thermostatic effect is not necessarily undesirable, as it may prevent overheating of the container and the food. However, two nominally similar susceptors may break up at substantially different temperatures due to manufacturing variances. Moreover, the entire surface of the susceptor does not necessarily break up uniformly or at the same time, thus introducing a further element of unpredictability. It is this unpredictable and mostly uncontrolled nature of the breakup that is undesirable. Furthermore, it is undesirable to permit the formulation of large cracks in the interactive layer, since it is these large cracks that are reflected in the substrate, causing the susceptor to lose its structural integrity.
It will thus be appreciated that there is a need for an improved susceptor that can be readily mass-produced and has an enhanced and predictable ability to resist cracking and breakup.